Rex Grigg's Lux Per Square Inch

[geowashlaw]

Anyone understand Rex Grigg's Lux Per Square Inch?

My tank surface area is 530 square inches.

My lux output on my T5 bulbs is 314.

I don't know what it is on the T8 bulbs.

Here is how he says to figure LSI.

"Now I know what you are thinking. I can do watts per gallon pretty easy. But how do I figure this LSI thing? Well find the surface area of your tank in square inches. Then take the LSI number you want to reach. Multiply the surface area by the LSI. Take the result and divide it by the LUX rating of your chosen light. I will admit that this system is not perfect by any means. But I think it gives a better idea than the WPG rule"

This it doesn't seem right to me because you are putting in the LSI you want to reach and I'm not sure what that gives you. Is he solving for the wattage that you need to achieve that LSI? When I run his formula using 30 LSI, I get approx. 50.5. So, is that how many watts I would need to get the 30 LSI?

If that is what he is figuring, are any of you smarter than me and can tell me how to figure out the LSI that I actually have?

Here is what he gives as the ranges for low light to very high light.

Low light is 12-17 LSI — you can grow a fair amount of aquarium plants.
Medium light is 20-25 LSI — you can grow most of the aquarium plants available.
High light is 28-32 LSI — you can grow all aquarium plants.
Very high light is over 35 LSI — same as high — all doubts removed.

[Fishalicious]

Wanted LSI X Surface Area = Result / your Lux

Soooooooo

30 x 314 = 9420 / 314 = 30

So your result is LSI 30 according to what his calculations are - High Light

[ILuvMyGoldBarb]

Close Jess, but not quite. :) You forgot his surface area in the equation.

First thing you have to know is your target. You need to know if you are going for a low, medium, or high light tank. Lets say your goal is to have a high light tank, so your target LSI is going to be at least 28. Next thing you need is your surface area, and in your case that happens to be 530.

So now you have 28x530=14840

No you have to divide your result by the Lux rating of the bulbs you have, and that is 314.

No you have 14840/314

That leaves you with a LSI of 47.26. You overshot your goal. :)


In this equation your Target, and your surface area are going to be your static values, and the LUX of your chosen bulb will be your changing value.

Here is the big problem with his equation, you don't need to have a target to figure it out. You just need to know the surface area of your tank and the Lux of your bulbs. Your Target value doesn't mean anything to you until you have your actual value, it is simply a reference. According to his equation, to hit your target LSI you need a bulb that has a LUX that equals the surface area of your tank. Rex might be a smart guy but I think he's missed the mark on this one. So, the whole equation is really useless, it will not give you the values you want. While I agree with him that the WPG idea is quite imperfect, I don't believe LSI is the answer either. The answer lies solely in PAR, but the fact is, for the average planted tank enthusiast, it is as simple as getting the right color temperature with the proper CRI and with enough output to grow what they want.

[Fishalicious]

My equation was right I just used the Lux instead of the surface area amount when calculating - ooops

[ILuvMyGoldBarb]

LOL, you are cute Jess. LOL

[Fishalicious]

So is it safe to say you have a soft spot for dumb blondes ILMGB??

[geowashlaw]

So, the whole equation is really useless, it will not give you the values you want.

That's kinda what I was thinking. I took some algebra and calculus a loooong time ago and I don't remember a whole lot, but I think I remember enough to know that his equation doesn't actually tell you anything useful.

The 314 is for one T5 bulb. I have two. So do you use 628? Also, I have two T8 6700k bulbs that are (after much internet research) 80 lux each, so 160. So total lux then would be 788. But in watts, its only 2.75w/g.

Simple math tells me that for lux per square inch, you would just divide total lux by square inches and that his introduction of the user chosen variable of what LSI you want to achieve throws off his equation.

I think his idea that lux may be a good indicator of light needed is possibly correct. I just think his math suffers from some logic flaws. Maybe some of you more experienced plant keepers could actually refine his idea into something more useable.

[ILuvMyGoldBarb]

So is it safe to say you have a soft spot for dumb blondes ILMGB??

Not dumb blonds. But you're not dumb. :)

[digital3]

Yeah... I never used the whole LUX equation thing. Sorry I never answered your initial inquiry about it, geo. Just got back on today.

[geowashlaw]

No problem ... I ran some test numbers with his formula and you can make it work out to whatever you want just by picking the right desired LSI, so I'm fairly certain his formula is wrong, even if his idea is right.